Provided you get the right amp for the job (power/impedance), you can safely say that at pretty much any point. With the exception of some deliberately non-linear amps, such as tubes.For example, If you have heard 10-15 amps, when can you safely say that the 16th one you try isn't going to bring any difference of a flavor that the last ones you have heard within that variation ?
That depends. I listen to a wide variety of music, occasionally a piece deliberately has a shrill treble, a bass that isn’t fat, a mid range that isn’t supposed to be “liquid deep” or meld with bass and treble. I personally don’t want a system that tries to force these traits on to a composition that deliberately isn’t supposed to have them. I just want a high fidelity system, IE. One that doesn’t attempt to change the composition/recording. That’s nirvana for me personally.When u get both, and no shrill treble or fat bass and a liquid deep midrange that melds with bass and treble, then my son/daughter you have attained nirvana and no more searching or analyzing is needed.
G